1. Field of the Invention
The present invention relates to a structure for reducing noise, and more particular, to a structure for reducing noise of a ventilating fan.
2. Description of the Related Art
FIG. 1 shows a known type of a ventilating fan 10. Typically, a plate 12 having orifices (hereinafter orifice plate 12) which are located below a scroll casing 11 of the ventilating fan is employed to reduce noise of the ventilating fan 10. The orifice plate 12 has the same size as that of an opening 14 of a housing 13 of the ventilating fan, and is configured to cover the opening 14. The orifice plate 12 is fixed with the scroll casing 11 by screws.
The noise-reduction structure of the ventilating fan in the prior art is problematic in the following aspects: 1) since an area of the orifice plate 12 is large and thus a space formed by it with the scroll casing 11 is also large, the suctioned air will generate turbulence in the space, thereby creating noise; 2) since the area of the orifice plate is so large, it necessitates more materials to manufacture it, therefore thus its cost is very high; and 3) because the orifice plate 12 has such a large area, a plurality of screws are needed to fix it, and this results in more mounting time and low efficiency in operation.
Typically, as shown in FIG. 2, when the ventilating fan incorporating the noise-reduction structure in the prior art operates in a frequency zone of 500 Hz-1000 Hz, the noise generated is in the range from 14 to 23 dB.